Automatic, step ratio change transmissions having plural planetary gear sets providing a plurality of speed ratios are well known. Vehicle drivetrains powered by internal combustion engine type prime movers have long employed such transmissions and, of late, as disclosed in U.S. Pat. No. 4,296,650, such transmissions have been proposed for drivetrains powered by electric motor type prime movers of the AC and DC type. Such transmissions, when powered by either type of prime mover, improve vehicle driveability and drivetrain efficiency. Vehicle driveability is affected by many factors. Three important driveability factors are transmission shift quality, vehicle gradability, and vehicle acceleration. Driveability and efficiency improvements are, of course, directly related to efficiency of the transmission, ratio coverage of the transmission, size of the ratio steps, and shift schedule of the ratio step changes. Transmission efficiency is improved by minimizing gear meshes and spin speeds, by minimizing rotating clutches and clutch spin speeds, by avoiding power recirculation within the qeartrain, and by providing ratio steps which allow the electric motor to operate most of the time in its most efficient speed range.
Avoidance of power recirculation within the transmission reduces geartrain power requirements and therefore minimizes the design size of many transmission components such as gears, shafts, clutches/brakes, bearings, etc.
It is also known to employ relatively high speed AC motors as prime movers in vehicle drive-trains. Such motors are durable and substantially maintenance free. Further, the power density and efficiency of such motors are relatively high when they are properly designed and controlled. Such motors may operate at about 10,000 RPM and their torque/speed characteristics require at least a two-speed transmission with a shiftable ratio step of around 2:1 and a larqe overall speed reduction. Such a ratio step is not readily available from known simple planetary gear sets. More complex qear sets such as the Ravigneaux or Simpson gear sets have been proposed. These, however, have the disadvantages of relatively low overall speed reductions, complex componetry, and rotating clutches. Hence, they require additional speed reduction which adds to complexity and cost and which further reduces efficiency.